Electrochemical fuel cells convert reactants, namely fuel and oxidants, to generate electric power and reaction products. Electrochemical fuel cells generally employ an electrolyte disposed between two electrodes, namely a cathode and an anode. The electrodes both comprise an electrocatalyst disposed at the interface between the electrolyte and the electrodes to induce the desired electrochemical reactions. The fuel fluid stream which is supplied to the anode may be a gas such as, for example, substantially pure hydrogen or a reformate stream comprising hydrogen. Alternatively, a liquid fuel stream such as, for example, aqueous methanol may be used. The oxidant fluid stream, which is supplied to the cathode, typically comprises oxygen, such as substantially pure oxygen, or a dilute oxygen stream such as air.
Solid polymer fuel cells employ a solid polymer electrolyte, otherwise referred to as an ion exchange membrane. The membrane is typically interposed between two electrode layers, forming a membrane electrode assembly (“MEA”). While the membrane is typically selectively proton conductive, it also acts as a barrier, isolating the fuel and oxidant streams from each other on opposite sides of the MEA. The MEA is typically disposed between two plates to form a fuel cell assembly. The plates act as current collectors and provide support for the adjacent electrodes. The assembly is typically compressed to ensure good electrical contact between the plates and the electrodes, and to ensure adequate sealing between fuel cell components. A plurality of fuel cell assemblies may be combined in series or in parallel to form a fuel cell stack. In a fuel cell stack, a plate may be shared between two adjacent fuel cell assemblies, in which case the plate also serves as a separator to fluidly isolate the fluid streams of the two adjacent fuel cell assemblies.
Fuel cell plates known as fluid flow field plates have open-faced channels formed in one or both opposing major surfaces for directing reactants and/or coolant fluids to specific portions of such major surfaces. The open-faced channels also provide passages for the removal of reaction products, depleted reactant streams, and/or heated coolant streams. For an illustration of a fluid flow field plate, see, for example, U.S. Pat. No. 4,988,583, issued Jan. 29, 1991. Where the major surface of a fluid flow field plate faces an MEA, the open-faced channels typically direct a reactant across substantially all of the electrochemically active area of the adjacent MEA. Where the major surface of a fluid flow field plate faces another fluid flow field plate, the channels formed by their cooperating surfaces may be used for carrying coolant for controlling the temperature of the fuel cell.
Fluid flow field plates may also have apertures therein. For example, fluid flow field plates may have manifold openings for supplying and exhausting reactants and/or coolant to and from the channels. When assembled in a fuel cell stack, such manifold openings in the plates of adjacent fuel cells cooperate to form the manifolds for supplying and exhausting reactants and/or coolant to and from the stack.
Conventional methods of fabricating fluid flow field plates require the engraving or milling of flow channels (and optionally apertures) into the surface of rigid plates formed of graphitized carbon-resin composites. These methods of fabrication place significant restrictions on the minimum achievable cell thickness due to the machining process, plate permeability, and required mechanical properties. Further, such plates are expensive, both in raw material costs and in machining costs. The machining of channels and the like into the graphite plate surfaces causes significant tool wear and requires significant processing times.
Alternatively, fluid flow field plates can be made by a lamination process, as described in U.S. Pat. No. 5,300,370, issued Apr. 5, 1994, in which an electrically conductive, fluid impermeable separator layer and an electrically conductive stencil layer are consolidated to form at least one open-faced channel. Such laminated fluid flow field assemblies tend to have higher manufacturing costs than single-layer plates, due to the number of manufacturing steps associated with forming and consolidating the separate layers.
Alternatively, fluid flow field plates can be made from an electrically conductive, substantially fluid impermeable material that is sufficiently compressible or moldable so as to permit embossing. Expanded graphite sheet is generally suitable for this purpose because it is relatively impervious to typical fuel cell reactants and coolants and thus is capable of fluidly isolating the fuel, oxidant, and coolant fluid streams from each other; it is also compressible and embossing processes may be used to form channels in one or both major surfaces. For example, U.S. Pat. No. 5,527,363, issued Jun. 18, 1996, describes fluid flow field plates comprising a metal foil or sheet interposed between two expanded graphite sheets having flow channels embossed on a major surface thereof.
However, forming apertures in expanded graphite sheet can be problematic, especially in the context of high-volume manufacturing. Expanded graphite sheet is formed from expanded graphite particles compressed into thin sheets. The sheet comprises aligned expanded graphite particles and constituent layers of carbon atoms parallel to the surface of the sheet. A conventional approach to making apertures in the sheets involves punch cutting the apertures, in which mating male and female punch features shear or tear the expanded graphite sheet. Punch cutting may be performed during embossing or in a separate step following embossing. Due to the nature of the material, however, punch cutting typically results in apertures with poor or inconsistent edge quality, material flaking around the edge of the aperture, and material cutout failures. In addition, where punch cutting occurs in a separate step following embossing, inconsistently placed apertures can also be a problem.
It would be desirable to have a method and apparatus for forming apertures in expanded graphite sheet flow field plates having improved edge characteristics, in an efficient and repeatable manner.